Photoelectric conversion devices have been used in various fields such as light-receiving sensors and solar cells. Among these, the solar cells have drawn public attention as one of the energy sources that are friendly to the earth. Moreover, together with increased concerns about environmental-related issues in recent years and accelerating policies for introducing solar cells in various countries, solar cells have been widely used rapidly.
In recent years, in order to achieve both of reduced costs and high efficiency of the photoelectric conversion devices, public attention has been drawn to a thin-film photoelectric conversion device which requires less raw materials, and developments thereof have been vigorously carried out. In particular, a method for forming a good-quality semiconductor layer on an inexpensive base member such as a glass plate by using a low-temperature process has been expected as a method for achieving a photoelectric conversion device at low costs.
In the case of manufacturing a thin-film photoelectric conversion device for power with a large area capable of generating a high output with a high voltage, a structure in which a plurality of thin-film photoelectric conversion devices, each formed on a base member, are serially-connected to one another with wiring, is not used, but a structure in which, in order to provide a good yield, a thin-film photoelectric conversion unit layer formed on a large base member is divided into a plurality of cells so that these cells are serially-connected to one another by the use of patterning to be integrated, is generally used. For example, in the case of a thin-film photoelectric conversion device of a type that allows light to be made incident on the glass base member side, in order to reduce electrical losses due to a resistance of the transparent electrode layer on the glass base member, separation grooves that divide the transparent electrode into a plurality of strip shapes having a predetermined width are formed by using a laser scribing process, and the respective cells are serially-connected to one another in a direction perpendicular to the longitudinal direction of the strip shapes so as to be integrated.
Moreover, in order to form a thin-film photoelectric conversion device, it is indispensable to provide a transparent electrode layer as one portion thereof. That is, the thin-film photoelectric conversion device includes one or more photoelectric conversion units between the transparent electrode layer and a back electrode layer. In this case, light is made incident on the transparent electrode layer side. As the transparent electrode layer, for example, conductive metal oxides such as SnO2 and ZnO are used, and the layer is formed by using a CVD method, a sputtering method, a vapor-deposition method, or the like. The transparent electrode layer is desirably provided with fine surface unevenness so as to have such an effect for increasing scattering of the incident light.
The photoelectric conversion unit is formed of semiconductor layers having a pn junction or a pin junction. In the case where the photoelectric conversion unit has the pin junction, a p-type layer, an i-type layer and an n-type layer are stacked in this order or in a reversed order thereof, and a unit including an amorphous layer as the i-type photoelectric conversion layer that occupies a main portion of its unit is referred to as an amorphous photoelectric conversion unit, and a unit including a crystalline layer as the i-type layer is referred to as a crystalline photoelectric conversion unit. An amorphous silicon layer or a crystalline silicon layer serving as a silicon-based thin film may be used as the semiconductor layer, and a thin film made of CuInSe2 (abbreviated as CIS) or CdTe may be used as the compound semiconductor thin film. Note that, in the present specification, the terms “crystalline” and “microcrystalline” also mean those materials partially containing amorphous components.
A photoelectric conversion unit, included in the silicon-based thin-film photoelectric conversion device, has a pin junction formed of a p-type layer, an i-type photoelectric conversion layer substantially made of an intrinsic semiconductor, and an n-type layer. In the case where the i-type layer is made of amorphous silicon, the photoelectric conversion unit is referred to as an amorphous silicon photoelectric conversion unit, and in the case where the i-type layer is made of silicon containing crystalline components, the photoelectric conversion unit is referred to as a crystalline silicon photoelectric conversion unit. As the amorphous or crystalline silicon-based materials, not only materials containing only silicon as their main element, but also alloy materials containing carbon, oxygen, nitrogen, germanium and the like as their main elements may be used. Moreover, the conductive layer does not necessarily need to be made of the same main element as that of the i-type layer, and for example, amorphous silicon carbide may be used as the p-type layer of the amorphous silicon photoelectric conversion unit, and microcrystalline silicon (also referred to as “μc-Si layer”) may be used as the n-type layer thereof.
As the back electrode layer to be formed on the photoelectric conversion unit, for example, a metal layer, such as Al or Ag, may be formed by using a sputtering method or a vapor deposition method. In general, a conductive oxide layer, such as ITO, SnO2 or ZnO, is formed between the photoelectric conversion unit and the metal electrode layer.
In the amorphous silicon thin-film photoelectric conversion device, the initial photoelectric conversion efficiency is lower than that of a photoelectric conversion device that utilizes monocrystalline or polycrystalline silicon, and a problem exists in that the conversion efficiency is lowered due to a photodegradation phenomenon when irradiated with light for a long period of time. Therefore, a crystalline silicon thin-film photoelectric conversion device that utilizes a crystalline silicon thin-film, such as polycrystalline silicon and microcrystalline silicon, as its photoelectric conversion layer has been expected and developed as a device that can achieve both of a reduced production cost and high photoelectric conversion efficiency. The reason for this is because the crystalline silicon thin-film photoelectric conversion device can be formed by utilizing a low-temperature plasma enhanced CVD in the same manner as in the amorphous silicon thin-film photoelectric conversion layer and because the crystalline silicon thin-film photoelectric conversion layer hardly causes any photodegradation phenomenon. Moreover, in comparison with the amorphous silicon photoelectric conversion layer that can photoelectrically convert light having wavelengths up to about 800 nm on the long-wavelength side, the crystalline silicon photoelectric conversion layer can photoelectrically convert light having longer wavelengths up to about 1200 nm.
Moreover, as a method for improving the conversion efficiency of the thin-film photoelectric conversion device, there has been known a method in which two or more photoelectric conversion units are stacked so as to form a stacked-type thin-film photoelectric conversion device. In this method, a front unit including a photoelectric conversion layer having a large energy-band gap is placed on a light incident side of a thin-film photoelectric conversion device, and behind this front unit, rear units having smaller band gaps are successively placed so that a photoelectric conversion process can be carried out over a wide wavelength range of incident light; thus, the conversion efficiency of the entire device can be improved. Among stacked-type thin-film photoelectric conversion devices, those in which an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit are stacked are referred to as hybrid-type thin-film photoelectric conversion devices.
In the thin-film photoelectric conversion device as described above, it becomes possible to make the photoelectric conversion layer thinner in comparison with a conventional photoelectric conversion device in which a bulk monocrystalline or polycrystalline silicon substrate is utilized; however, a problem is raised in that light absorption is limited by the film thickness. Therefore, in order to more effectively utilize light that is made incident on the photoelectric conversion unit including photoelectric conversion layers, the surface of a transparent electrode layer or a metal layer that is in contact with the photoelectric conversion unit is formed (textured) into a roughened surface with fine unevenness. That is, after light has been scattered on the interface with fine unevenness, the light is made incident onto the photoelectric conversion unit so that the optical path inside the photoelectric conversion layer is made longer; thus, an attempt is made to increase the quantity of light absorption. This technique for forming the surface unevenness (surface texture) is also referred to as a “light confinement” technique, and this forms an important basic technique so as to put the thin-film photoelectric conversion device having high photoelectric conversion efficiency into practical use.
In this case, in order to find surface uneven shapes of a transparent electrode layer optimally used for the thin-film photoelectric conversion device, an index that can quantitatively indicate the surface uneven shapes has been demanded. As the index for indicating the surface uneven shapes, a haze ratio, a surface area ratio (Sdr), or the like, has been known.
The haze ratio, which represents an index used for optically evaluating surface unevenness of a transparent plate, is indicated by (diffuse transmittance/total light transmittance)×100 [%] (JIS K7136). With respect to measurements of the haze ratio, a haze meter capable of carrying out automatic measurements is commercially available, and the measurements are easily executed. In general, an illuminant C is used as a light source for the measurements.
The surface area ratio is an index capable of indicating not only height differences in the unevenness, but also shapes of the unevenness in a manner so as to be included therein. Since fluctuations of the surface unevenness of the transparent conductive film are sharpened, the open circuit voltage and fill factor of the thin-film photoelectric conversion device tend to be lowered; therefore, the surface area ratio can be effectively used as an index for indicating the surface unevenness of a transparent conductive film for a thin-film photoelectric conversion device. The surface area ratio is also referred to as “Developed Surface Area Ratio”, and “Sdr” is used as its abbreviation. The Sdr is defined by equation 1 and equation 2 (K. J. Stout, P. J. Sullivan, W. P. Dong, E. Manisah, N. Luo, T. Mathia: “The development of methods for characterization of roughness on three dimensions”, Publication no. EUR 15178 EN of the Commission of the European Communities, Lucembourg, 1994).
                              S          dr                =                                                            (                                                      ∑                    j                                          M                      -                      1                                                        ⁢                                                                          ⁢                                                            ∑                      k                                              N                        -                        1                                                              ⁢                                                                                  ⁢                                          A                      jk                                                                      )                            -                                                (                                      M                    -                    1                                    )                                ⁢                                  (                                      N                    -                    1                                    )                                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                X                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                Y                                                                    (                                  M                  -                  1                                )                            ⁢                              (                                  N                  -                  1                                )                            ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              X              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              Y                                ×          100          ⁢          %                                    (                  Equation          ⁢                                          ⁢          1                )            
In this case, Ajk is indicated by the following equation 2:
                              A          jk                =                                            1              2                        ⁡                          [                                                                                          Δ                      ⁢                                                                                          ⁢                                              Y                        2                                                              +                                          {                                                                        Z                          ⁡                                                      (                                                                                          x                                j                                                            ,                                                              y                                k                                                                                      )                                                                          -                                                                              Z                            (                                                                                          x                                j                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      }                                                    2                                                                                                                    +                                                                            Δ                      ⁢                                                                                          ⁢                                              Y                        2                                                              +                                          {                                                                        Z                          ⁡                                                      (                                                                                          x                                                                  j                                  +                                  1                                                                                            ,                                                              y                                k                                                                                      )                                                                          -                                                                              Z                            (                                                                                          x                                                                  j                                  +                                  1                                                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      }                                                    2                                                                                                                                ]                                ×                                    1              2                        ⁡                          [                                                                                          Δ                      ⁢                                                                                          ⁢                                              X                        2                                                              +                                          {                                                                        Z                          ⁡                                                      (                                                                                          x                                j                                                            ,                                                              y                                k                                                                                      )                                                                          -                                                                              Z                            (                                                                                          x                                                                  j                                  +                                  1                                                                                            ,                                                              y                                k                                                                                      }                                                    2                                                                                                                    +                                                                            Δ                      ⁢                                                                                          ⁢                                              X                        2                                                              +                                          {                                                                        Z                          ⁡                                                      (                                                                                          x                                j                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      )                                                                          -                                                                              Z                            (                                                                                          x                                                                  j                                  +                                  1                                                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      }                                                    2                                                                                                                                ]                                                          (                  Equation          ⁢                                          ⁢          2                )            
In this case, ΔX and ΔY respectively indicate distances of measured intervals in the X direction and Y direction.
That is, Sdr indicates a rate of an increase of a surface area relative to an area on a flat XY plane. In other words, the Sdr value becomes greater, as the height differences in the surface unevenness become greater so that each convex portion is made as sharpened as possible.
In a conventional amorphous silicon thin-film photoelectric conversion device, a tin oxide (SnO2) film having surface unevenness is often used as a transparent electrode layer to be formed on a transparent base member such as a glass plate. The surface unevenness of this transparent electrode layer effectively contributes to confining light into the photoelectric conversion layer. However, although it is preferable to increase the surface unevenness so as to further enhance the light confinement effect, only the use of the SnO2 film solely makes it difficult to remarkably change the shapes of the surface unevenness, while light-transmitting property and conductivity required for the photoelectric conversion device are being properly maintained.
Moreover, in the case where an SnO2 film is formed on a glass plate as the transparent electrode layer having surface unevenness that is effective for the light confinement by using a normal-pressure thermal chemical vapor deposition method (normal-pressure thermal CVD method), since the thermal CVD method uses a high-temperature process in a range of about 550 to 650° C., a problem is raised in that the formation cost of the transparent electrode layer becomes high. Moreover, in the case where the film-forming temperature is high, another problem is that it becomes difficult to use inexpensive base members, such as normal glass plates and plastic films. Furthermore, when a reinforced glass plate is subjected to a high temperature process, its reinforcing effect is lost. Consequently, in the case where a base member of a glass plate is applied to a solar cell with a large area during a high-temperature process, the thickness of the glass plate needs to be increased so as to maintain its strength, causing a problem in that the glass plate becomes heavier.
Moreover, since the SnO2 film is poor in plasma resistant property, the SnO2 film may be reduced under a high plasma concentration containing hydrogen in a deposition environment of the photoelectric conversion layer. Since the SnO2 film is blackened when reduced, incident light is absorbed by the blackened electrode layer, with the result that the quantity of transmitted light into the photoelectric conversion layer is reduced to cause a reduction in the photoelectric conversion efficiency.
On the other hand, zinc oxide (ZnO) is less expensive than SiO2 or indium-tin oxide (ITO) that is widely used as a material for the transparent electrode layer, and is also advantageous in having high plasma resistant property so that it is desirably used as a material for the transparent electrode layer contained in a thin-film solar cell. In particular, in the case of a crystalline silicon thin-film photoelectric conversion device including a crystalline silicon layer, such as a thin-film polycrystalline silicon layer and a microcrystalline silicon layer, that consumes a large amount of hydrogen in comparison with deposition conditions of an amorphous silicon layer, and requires a high plasma concentration, as one portion of a photoelectric conversion unit, it is effective to use zinc oxide (ZnO) as the material for the transparent electrode layer.
Patent Document 1 (Japanese Patent Laid-Open Publication No. 2003-243676) has disclosed a structure in which an underlying layer containing insulating fine particles and a binder is formed on a glass plate so as to form a transparent electrode layer having a high light confinement effect at a low cost, and by allowing the insulating fine particles to occupy an area of 80% or more of the underlying layer, the surface unevenness of the transparent electrode layer to be formed on the underlying layer can be increased. Silica (SiO2) having a particle size of 0.1 to 1 μm is used as the insulating fine particles, and a silicon oxide is used as the binder. More specifically, the underlying layer is formed by a sol-gel method using a roll coater, and a ZnO layer is formed as the transparent electrode layer by using a sputtering method.